The development of high energy cell systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly active anode materials, such as lithium, calcium, sodium and the like, and the efficient use of high energy density cathode materials, such as FeS.sub.2, Co.sub.3 O.sub.4, PbO.sub.2 and the like. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. Therefore, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, it is necessary to use a non-aqueous electrolyte system.
One of the major disadvantages of employing lead dioxide as the active cathode material in a non-aqueous electrolyte system is that it will discharge at two different potentials. The first step in the discharge curve is attributed to the reduction of the lead dioxide to lead monoxide, while the second step is attributed to the reduction of the reaction product lead monoxide. Contrary to lead dioxide, lead monoxide will discharge in a non-aqueous cell system at a unipotential level. One advantage in employing a lead dioxide as the cathode material over lead monoxide is that it has almost double the capacity of lead monoxide. Thus in a non-aqueous electrolyte system, lead monoxide will have the advantage of discharging at a unipotential plateau with the disadvantage of having a relatively low capacity while lead dioxide will have the advantage of having a relatively high capacity with the disadvantage of discharging at two distinct voltage plateaus.
Many cell or battery applications, particularly in transistorized devices such as hearing aids, watches and the like, require a substantial unipotential discharge source for proper operation and, therefore, cannot use the dual voltage level discharge which is characteristic of non-aqueous lead dioxide cells. This dual voltage level discharge characteristic is similar to the dual voltage discharge characteristic of aqueous alkaline divalent silver oxide cells. Although many approaches have been proposed for obtaining a unipotential discharge from an aqueous alkaline divalent silver oxide cell, the approaches are not needed when lead dioxide is employed in an aqueous electrolyte cell system. Specifically, in an aqueous electrolyte cell system, lead dioxide will discharge almost entirely at its higher voltage level so that, in effect, the cell will produce a substantially unipotential discharge over the useful life of the cell. Contrary to this, when lead dioxide is used as the cathode material in a non-aqueous electrolyte system, the cell will discharge at a first potential for a significant time period and then decrease to a distinct lower potential for the remainder of the discharge. A problem usually encountered in various cell systems is that although an electrode-couple can function in an aqueous electrolyte, it is practically impossible to predict in advance how well, if at all, it will function in a non-aqueous electrolyte. Thus a cell must be considered as a unit having three parts -- a cathode, an anode and an electrolyte -- and it is to be understood that the parts of one cell may not be predictably interchangeable with parts of another cell to produce an efficient and workable cell.
A French Pat. No. 2,288,401 published on June 18, 1976 (counterpart to German application No. 2,545,498 published on Apr. 27, 1976) discloses a non-aqueous cell which employs a negative electrode, such as lithium, a non-aqueous-solvent electrolyte and a positive active electrode consisting of a positive active material of the oxides and oxidizing salts, the discharged reduction of which leads to metals of the group including lead, tin, gold, bismuth, zinc, cadmium and their alloys and an electronic conductor consisting at least on the surface of a material selected from the group including lead, tin, gold, bismuth, zinc, cadmium and their alloys. Several examples are disclosed in this reference in which lead monoxide is employed as the positive active material and lead, tin or graphite is employed as the electronic conductor. Although this reference teaches one means for obtaining a unipotential discharge for certain non-aqueous cell systems, as, for example, a cell employing lead monoxide as the positive active material , the subject invention is directed to the use of lead dioxide particles having a lead monoxide outer layer as the positive active material of a non-aqueous cell. The positive active material of this invention could also be expressed as lead monoxide particles having a lead dioxide core.
Accordingly, it is the primary object of this invention to provide a non-aqueous lead oxide cell which employs a positive electrode comprising lead dioxide particles each having a lead monoxide outer layer, and which has a substantially unipotential discharge voltage.
Another object of this invention is to provide a non-aqueous lead oxide cell which employs a lithium anode and a positive cathode composed of lead dioxide particles each having a lead monoxide outer layer, and which has a substantially unipotential discharge voltage.
Another object of this invention is to provide a non-aqueous lead oxide cell which employs a positive electrode composed of lead dioxide particles each having a lead monoxide outer layer, and wherein said lead monoxide varies between about 1 percent and 60 percent by weight of the lead oxides.